As a nasty primary drags down Mitt Romney, Florida U.S. Sen. Marco Rubio stepped in late Wednesday with a surprise endorsement that has fueled speculation he is seeking a spot on the ticket.

Rubio’s endorsement comes one week after a political mentor of his, former Gov. Jeb Bush, endorsed Romney and asked Republicans to get behind the former Massachusetts governor.

Both Rubio and Bush avoided making endorsements during the Jan. 31 Republican primary in Florida, though Rubio helped subtly to provide Romney support by condemning an immigration ad run by Romney rival Newt Gingrich.

But as the GOP primaries have dragged on, Gingrich and then Rick Santorum kept pounding away at Romney as he stumbled from one campaign gaffe to another. The result: Obama is now beating Romney in Florida, according to the latest Quinnipiac University poll that shows the president has momentum here and in Ohio and Pennsylvania, other crucial swing states.

“I don’t have a problem with primaries,” Rubio told Fox News Channel’s Sean Hannity Wednesday night. “But I think we’re at a stage now where at least two of the candidates have openly admitted that the only way they’re going to be able to win the nomination is to have a floor fight in Tampa in August.

“I don’t think there’s anything good about that. There is no way that anyone can convince me that having a floor fight at the convention in Tampa in August is a recipe for victory in November. On the contrary, I think it’s a recipe for disaster. So I just don’t think that’s a wise route to go.”

Rubio pointed out that Romney was “successful in the private sector” and had experience as a governor, and noted that home prices are down and gas prices are on the rise along with the national debt. Rubio also said “unemployment is up.” However, the rate is now declining and the recession began under President George Bush, whose father and former president George H.W. Bush also endorsed Romney.

Hannity asked the obvious question: Would Rubio leave the door open “if a presidential candidate calls on you” to run on his ticket.

Rubio didn’t say “no.” But he didn’t say “yes,” either.

Rubio said he is focused on being a Florida senator.

“I don’t believe I’m going to be asked to be the vice presidential nominee,” Rubio said. “That’s not what I intend to be. That’s not what I want to be. And that’s not what’s going to happen.”

Hannity pointed out that, at a recent event in Naples, Fla., Romney called Rubio a vice presidential candidate and that the crowd gave him a standing ovation. Rubio said he was flattered.

Conservatives have been pushing for Rubio for months. He brings a fresh face, a Hispanic name and an electrifying speaking style to the campaign trail. Jeb Bush likes him.

Like any career politician, Rubio has a past that could haunt him. When he ran the Florida House as its speaker, he paved the way for some cushy consulting contracts with area hospitals and, after his 2010 election to the U.S. Senate, his office misrepresented his parents’ flight from Cuba (they left Cuba under the rule of Fulgencio Batista, not Fidel Castro).

With bits of his biography slipping out, Rubio plans to release an autobiography this June — just as campaign season gears up.

Rubio exploded onto the national scene in the 2010 election when he chased then-Gov. Charlie Crist from the GOP and bested him in the Senate race. Rubio, a Hispanic tea party favorite, is seen by many Republicans as an exciting potential addition to a Romney ticket that needs excitement.

It’s unclear how much of a difference Rubio’s endorsement will make during a primary in which a sizable segment of the Republican electorate seems to have an instinctive aversion to the political establishment.

Still, at the moment, former Gov. Bush is a slightly better choice for Romney’s running mate. A recent McClatchy-Marist poll found that the Obama-Joe Biden ticket ties Romney-Bush at 47-47 percent — but it leads Romney-Rubio at 49-44 percent among registered voters.

Rubio said he is not interested in the type of politics that, he says, Obama engages in.

“He’s going to deliberately divide the American people against each other in a calculated effort to get to 50 percent plus 1,” Rubio said.

Rubio indicated that he knew some conservatives would be disappointed with his involvement in the primary, but he said it had to happen.

“No. 1, Mitt Romney will govern as a conservative,” he said. “And No. 2, that he will be head and shoulders better than the guy who’s in the White House right now.”